I guess the archer will be left out.
Here is why: Imo they have spoiled the possible other two classes already. When you read the info in the “World” section of the D4 homepage they tell you about regions with druids, barbarians and magicians (= sorceress)
They also show the “homeworlds” of monks, witches and cultists. So I would say these are the three classes which they are considering for the two left spots.
I would go with the monk being safe (as the melee holy class) leaving the dark spellcaster slot for either a witch or a cultist.
I think the witch has a slight advantage over the cultist for being a second default female class, but a cultist has its arguments as well 
These choices would also fit in the “All time Diablo favorites” strategy they said D4 will be approaching:
- Babarian - Default Diablo character (male)
- Sorceress - Default Diablo character (female)
- Druid - D2 character
- Monk - D3 character
- Witch (or cultist) - New character